MUSIC (Multiple Signal Classification) method, ESPRIT (Estimation of Signal Parameters via Rotational Invariance Technique) method or the like are well known as conventional signal arrival direction estimating schemes.
The MUSIC method is an algorithm that estimates the directions of a plurality of radio signals which arrive at an array antenna, etc. at the same time, and in this method, the parameters of incoming signals are estimated using components of noise that has no relationship with the incoming signal. In estimating the arrival direction of the radio waves using the MUSIC method, eigenvalue s of a correlation matrix obtained from the received signals are determined and the determined eigenvalues are divided into signal eigenvalues and noise eigenvalues according to the number of incoming waves, and, a noise eigenvectors corresponding to the noise eigenvalues are determined, an angle spectrum is determined from the noise eigenvectors thus obtained, and, by averaging these out by the angle (in other words, the frequency) to determine a MUSIC spectrum, the arrival direction of the incoming signal is estimated. Also, the basic principle of the ESPRIT method is the same as that of the MUSIC method. However, with this technique, by arranging a plurality of pairs of arrays having parallel array surfaces, the geometric information and amount of calculations of the arrays, which are required in the MUSIC method, are reduced.
Techniques such as those described in Patent Document 1, Patent Document 2 and Patent Document 3, for instance, are proposed as improved schemes of these methods.
The technique described in Patent Document 1 realizes arrival direction estimation with a reduced amount of calculations, by using an inverse matrix of the input signal correlation matrix, instead of using eigen value decomposition. Also, the technique described in Patent Document 2 realizes a reduction in the amount of calculations by configuring a Toeplitz matrix only from the correlation vectors of a first receiver and another receiver and using this instead of the input signal correlation matrix. Also, the technique described in Patent Document 3 realizes arrival direction estimation, even when a signal source having a frequency that was not predicted at the time of deciding the array interval is present, by causing physically rotating the arrays. All of these technologies realize a reduction of the amount of calculations and improvement of performance, based on the assumption that the input signal correlation matrix is “non-singular” (i.e. it has an inverse matrix).    Patent Document 1: Japanese Patent Application Laid-Open No.2002-148324    Patent Document 2: Japanese Patent Application Laid-Open No.Hei11-133130    Patent Document 3: Japanese Patent Application Laid-Open No.2001-108734